1. Field of the Invention
The present invention relates to anode active materials, methods of preparing the same, and anodes and lithium batteries containing the materials. More particularly, the invention is directed to anode active materials having particular silicon peaks as measured by X-ray photoelectron spectrometry.
2. Description of the Related Art
In an effort to achieve high voltages and energy densities, research and development has been extensively conducted into non-aqueous electrolyte secondary batteries using lithium compounds as anodes. Specifically, metallic lithium has become the subject of intense research due to its ability to impart high initial battery capacity. Accordingly, lithium has gained great attention as a prominent anode material. However, when metallic lithium is used as an anode material, large amounts of lithium are deposited on the surface of the anode in the form of dendrites, which may degrade charge and discharge efficiencies or cause internal-shorts between the anode and the cathode (positive electrode). Further, lithium is very sensitive to heat or impact and is prone to explosion due to its instability, i.e., high reactivity, which has held up commercialization. In order to eliminate these problems with the use of metallic lithium, carbonaceous materials have been proposed for use as anode materials. Carbonaceous anodes perform redox reactions such that lithium ions in the electrolytic solution intercalate/deintercalate in the carbonaceous material which has a crystal lattice structure during charge and discharge cycles. These anodes are referred to as “rocking chair type” anodes.
The carbonaceous anode has made a great contribution to the widespread use of lithium batteries by overcoming various disadvantages associated with metallic lithium. However, electronic equipment are becoming smaller and more lightweight, and the use of portable electronic instruments is becoming more widespread, making the development of lithium secondary batteries having higher capacities a major focal point. Lithium batteries using carbonaceous anodes have low battery capacity because of the porosity of the carbonaceous anode. For example, graphite (which is an ultra-high crystalline material), when used in a LiC6 structure (made by reaction of graphite with lithium ions), has a theoretical capacity density of about 372 mAh/g. This is only about 10% that of metallic lithium, i.e., 3860 mAh/g. Thus, in spite of many problems with conventional metallic anodes, studies for improving battery capacity using metallic lithium as the anode material are actively being carried out.
A representative example of such studies is the use of materials that can alloy with lithium, e.g., Si, Sn, Al, or the like, as anode active materials. However, materials that can alloy with lithium, such as Si or Sn, may present several problems, including volumetric expansion during formation of the lithium alloy, creation of electrically disconnected active materials in an electrode, aggravation of electrolytic decomposition, and so on.
In order to overcome these problems with the use of such a metallic material, a technique of using a metal oxide exhibiting a relatively low volumetric expansion as an anode active material has been proposed. For example, use of an amorphous Sn-based oxide has been proposed which minimizes the Sn particle size and prevents agglomeration of Sn particles during charge and discharge cycles, thereby leading to improvement of capacity retention characteristics. However, Sn-based oxides unavoidably cause reactions between lithium and oxygen atoms, which is responsible for considerable irreversible capacities.
High capacity electrodes using silicon oxides as the anode materials for secondary lithium ion batteries have also been proposed. However, irreversible capacities are considerably large during initial charge-discharge cycling stages, giving the secondary lithium ion batteries undesirable cycling characteristics and preventing practical use.